The invention relates to a method which assists the manual preparation of samples on a sample support for ionization with matrix-assisted laser desorption and a corresponding deposition aid. Deposition aids have become known in the prior art particularly for use with microtitration plates. German utility model DE 20 2007 018 535 U1 describes a pipetting aid for transparent microtitration plates, which are placed into a base plate with the aid of an adapter. The base plate contains light sources, which are each assigned to an opening in the adapter and a cavity of the transparent microtitration plate. A switching or control unit activates the light sources independently of each other, and the illumination through the adapter and the transparent plate indicates where a sample liquid is to be pipetted. German utility model DE 20 2005 017 946 U1 concerns a similar structure.
In contrast to microtitration plates, sample supports for ionization with matrix-assisted laser desorption are generally opaque. This is a result of their electrical conductivity that serves to prevent static charges from forming on the sample support during the laser desorption. Electrical conductivity is fundamentally undesirable for microtitration plates because the cavities provide a larger interaction area with the sample liquid contained in the cavities, unlike MALDI sample supports with their flat sample sites, which are largely designed to be flush with the rest of the surface. This enlarged interaction area can—if it is conductive and the samples are liquid—promote undesirable boundary layer processes, for example the deposition of charge carriers dissolved in the liquid, such as salts, or chemical boundary layer reactions.
In a similar manner to the above-mentioned utility models, patent document U.S. Pat. No. 4,692,609 A describes a holder for a transparent microtitration plate, on whose base several light sources are arranged in such a way that they can illuminate a well of the plate from below in order to indicate to users where they are to pipette the liquid. Alternatively, the plate can be illuminated from the front by a guidable light source, although the patent document does not disclose a design for a guidable light source.
Publication WO 2007/038521 A1 shows an arrangement with a telescopic arm with a light source mounted at its end. The arm can be extended with the aid of an actuator so that the light source can be positioned vertically above each well of the microtitration plate to provide the illumination. This arrangement has the disadvantage that the light source itself, together with its holder, has to be moved each time it is positioned over a specific well, which imposes increased demands on the mechatronic actuators.
Further publications which deal with sample preparation on microtitration plates are FR 2 649 511, US 2005/0046847 A1, WO 83/00047 A1, WO 2007/071575 A1 and WO 2007/121324 A1.
Publication US 2002/0191864 A1 discloses the use of an image to identify the sample areas on the sample support which have been deposited with a sample, and to align the laser beam onto these areas. A method which assists the manual preparation of a MALDI sample support is not disclosed, however.
Publication EP 1 763 061 A2 concerns, among other things, the monitoring of deposition processes on MALDI sample supports with the aid of an imaging workstation.
Patent application laid open to inspection DE 10 2004 020 885 A1 is concerned with the preparation of samples of microbial origin on MALDI sample supports with the objective of automating the transfer of biological material from agar plates to sample sites on MALDI sample supports. To this end, agar plates are transported, via a conveyor belt, to a robot and set down on a 3D stage. An image processing system detects individual colonies on the agar plate and positions a sampling rod accordingly. An individual sampling rod is used for one single transfer only and is replaced afterwards. In order to take up biological material, the sampling rod is released from a holder and drops from a height of a few millimeters onto the colony. The contact with the colony thus achieved is designed to guarantee that only biological material adheres to the sampling rod, and no agar is transferred onto the MALDI sample support. If too much agar is transferred onto the MALDI sample support, the quality of the mass spectrometric identification is reduced because agar suppresses the signals of the characteristic protein ions. A fine sensor system to control the contact is not provided. The sampling rod does, however, vibrate, and it can be wetted with water before the sampling in order that a sufficient quantity of biological material from a colony adheres to the sampling rod and can be transferred onto a sample site of a MALDI sample support.
There is thus still a need to create an improved deposition aid for sample preparation on sample supports for ionization with matrix-assisted laser desorption.